Describe the principles behind the technique, method or instrument you have used.
This should include a brief outline of the scientific theory i.e. how does it work and why
The origin of flame colours
Flame tests are conducted to determine the identity of the metal present in a salt sample. Different metal cations produce different coloured flames, and their identity can be determined by looking up a reference table.
When the salt is heated, the electrons in the metal ions in the salt sample are 'excited', i.e. they gain energy and are promoted to a higher energy level in the ion's electronic structure. These electrons however, are not in a stable state due to their high energy and they consequently release that extra energy after a brief moment, and return back to a lower energy level. The energy released leaves the ions in the form of photons, i.e. light, which causes the salt sample to emit a coloured flame.
The higher the energy released, the higher the frequency of the light emitted, with the lowest frequency being red and the highest violet (or even ultraviolet), following the sequence of the colours of the rainbow.
Different amounts of energy are released depending on the electronic structure of the ion, which is unique to the element of that metal ion. Hence, each metal ion produces its own characteristic colour, which allows us to use the flame test to identify them.
Some common metal ions and their flame colours are listed below:
Lithium - red
Sodium- orange
Potassium - lilac
Calcium - brick-red
Barium - pale green/apple green
Copper (II) - bluish-green with white flame centre if heated at high temperature
Lead - greyish-white
Magnesium - white
Zinc - blue
Flame colours are produced from the movement of the electrons in the metal ions present in the compounds. For example, a sodium ion in an unexcited state has the structure 1s22s22p6.
When you heat it, the electrons gain energy and can jump into any of the empty orbitals at higher levels - for example, into the 7s or 6p or 4d or whatever, depending on how much energy a particular electron happens to absorb from the flame.
Because the electrons are now at a higher and more energetically unstable level, they tend to fall back down to where they were before - but not necessarily all in one go.
An electron which had been excited from the 2p level to an orbital in the 7 level, for example, might jump back to the 2p level in one go. That would release a certain amount of energy which would be seen as light of a particular colour.
However, it might jump back in two (or more) stages. For example, first to the 5 level and then back to the 2 level.
Each of these jumps involves a specific amount of energy being released as light energy, and each corresponds to a particular colour.
As a result of all these jumps, a spectrum of coloured lines will be produced. The colour you see will be a combination of all these individual colours.
The exact sizes of the possible jumps in energy terms vary from one metal ion to another. That means that each different ion will have a different pattern of spectral lines, and so a different flame colour.
Using silver nitrate solution to test for anions
Carrying out the test
The solution is acidified by adding dilute nitric acid. (Remember: silver nitrate + dilute nitric acid.) The nitric acid reacts with, and removes, other ions that might also give a confusing precipitate with silver nitrate.
The chloride, bromide and iodide precipitates are shown in the photograph:
The chloride precipitate is obviously white, but the other two aren't really very different from each other. You couldn't be sure which you had unless you compared them side-by-side.
All of the precipitates change colour if they are exposed to light - taking on grey or purplish tints.
The absence of a precipitate with fluoride ions doesn't prove anything unless you already know that you must have a halogen present and are simply trying to find out which one. All the absence of a precipitate shows is that you haven't got chloride, bromide or iodide ions present.
The chemistry of the test
The precipitates are the insoluble silver halides - silver chloride, silver bromide or silver iodide.
Hydrochloric Acid to test for carbonates
What Causes the Fizz?
Carbonate minerals are unstable in contact with hydrochloric acid. When acid begins to effervesce (fizz) on a specimen a reaction similar to the one shown below is taking place.
On the left side of this reaction the mineral calcite (CaCO3) is in contact with hydrochloric acid (HCl). These react to form carbon dioxide gas (CO2), water (H2O), dissolved calcium (Ca++) and dissolved chlorine (Cl--). The carbon dioxide bubbles that you observe are evidence that the reaction is taking place. When that occurs, calcite or another carbonate mineral is present.
Many other carbonate minerals react with hydrochloric acid. Each of these minerals consists of one or more metal ions combined with a carbonate ion (CO3--). The chemistry of these reactions is similar to the calcite reaction above. The mineral reacts with hydrochloric acid to produce carbon dioxide gas, water, a dissolved metal ion and dissolved chlorine. The reactions for magnesite (MgCO3) and siderite (FeCO3) are shown below.
The Barium Chloride Test for Sulfate (SO42-) Ions.
Any soluble sulphate will give a white precipitate of barium sulphate.
barium chloride + zinc sulphatezinc chloride + barium sulphate.
BaCl2(aq) + ZnSO4(aq) ZnCl2(aq) + BaSO4(s)
The ionic equation is Ba2+(aq) + SO42-(aq) BaSO4(s)
The test is made in the presence of dilute hydrochloric acid to remove any carbonate or sulfite ions
which may be present. These ions will also produce a precipitate which would confuse the results.
Describe any possible sources of error or contamination in the technique or process.
Cations tests.
Problems or contaminations that could occur:
- Inoculating loops are dirty
- Using concentrated Hydrochloric acid will produce volatile chlorides.
- Results are subjective as different people will interpret colours differently.
- Possibility that the pots of unknown samples could have been cross contaminated
- Metal salts fall into the Bunsen flame and therefore could give a false colour.
- Different colours observed if samples not held in the hottest part of Bunsen flame
Anion tests.
Problems or contaminations that could occur :
- Contamination of chemicals in bottles
- Cross contamination through pipettes
- Dirty glassware leading to contamination
- Difficult to distinguish if the precipitate was creamy coloured of white, giving false results
Describe how these sources of error or contamination in the technique or process can be minimised.
The inoculating loops need to be cleaned using hydrochloric acid and Bunsen burners before each test and a different inoculating loop could be used for each sample.
Unknown samples could be taken from a watch glass and not directly from the pot, with one inoculating loop assigned to each sample.
Ensure that the sample is held at the top of the blue cone to ensure that it is held in the hottest part of the flame.
Tip excess metal salt out of the Bunsen between tests.
Share results with the class to remove possibility of identifying colour of flame incorrectly.
Compare colour of flame against a standard
Ensure all glassware is clean before starting
Use a new/clean test-tube each test rather than just rinsing out test-tube.
Ensure each pipette is only used once
Told test tubes against a white tile to aid the identification of the colour.
Explain the possible consequences of the error or contamination.
Cations tests.
The colour will not be observed correctly and as a result the incorrect cation will be recorded.
Anion tests
The precipitate will either be the incorrect colour, or no precipitate will form and as a result the anion will not be correctly determined.
Suggest any improvements you could make to the technique, justifying your suggestions.
Improve techniques by repeating the practical to get more accurate results.
- Measure the amount of reactants that I was using to improve reliability.
- Repeat all investigations in order to eliminate error
- Some cations may have similar precipitates with the same anion, extend my practical by carrying out a flame test by placing a small sample of the precipitate in a Bunsen burner.
- For cations complete the chemical tests for further analysis that will help to determine the cations present. Some metal ions give coloured hydroxide precipitates that can be used as a simple identification test. Some metal ions give a white precipitate and others no precipitate at all. Adding excess sodium hydroxide or ammonia solution can sometimes give useful extra observations.
Use potassium iodide solution ==> yellow precipitate to determine the presence of Lead (II) ion
Use ion chromatography in the determination of anions. This is a quantative method rather than a qualitative method and therefore more accurate.